The great rivers head south to but India wants to dam and divert them to provide hydro electricty and water for its drought areas. It would be clean power but it could cut off some of Bangladesh's water

This week India promised to instal 20 GW of solar electricty by 2020. Some will come from developments like this solar village outside Calcutta. Houses here were snapped up in a few days Photograph: Bikas Rauniar

On a 1,000-mile journey from the Himalayas to the Bay of Bengal, the Guardian finds clear evidence of the terrible threat that global warming now poses to the millions who rely on water from the roof of the world